I would probably very politely express my opinion that they should get over it, and/or find a way to disassociate myself from a person who was so extremely invested in this demonstrably extremely fatal aeroplane that they felt the need to approach people to complain about it being mocked. Like, if they designed it, maybe there's a conversation to be had about that, but they probably agree that it was excessively fatal!
If it was a political, religious or moral concept, that's a little more complex and we can't really discuss that here, but I would generally avoid insulting those unless I found them notably vile.
But this is silly rule in an RPG, that I understand and used for decade, and I'm sorry but that's 100% fair game.
EDIT - Also, let's not pretend THAC0 is in any way necessary with or even really associated with OSR-type games. Almost every single one of them has ditched it. So it's not really associated with a particular style. It's pretending to suggest it is.